The present invention pertains generally to methods of preparing laser materials and of lasing those materials and in particular to methods of suitably preparing color-center materials and for achieving broadly tunable lasing in those materials.
Color-center lasers have been particularly useful for providing tunable lasing in the infrared and near infrared spectral regions. Examples of color-center lasers which have been reported are: (1) B. Fritz et al. Laser Effect in KCl with F.sub.A (Li) Centers in Solid State Communications 3(3): 61-68 (1965), (2) U.S. Pat. No. 3,970,960 issued 20 July 1976 to Linn F. Mollenauer, and (3) I. Schneider and M. J. Marrone Continuous-Wave Laser-action of (F.sub.2 +).sub.A Centers in Sodium-Doped KCl Crystals, Optics Lett. 4:390(1979). Color centers which have been made to lase include the F.sub.A center in several host crystals, such as KCl and RbCl, the F.sub.B center in KCl, and the F.sub.2.sup.+ center in LiF, KCl, NaCl, and KF.
Definitions of the above color centers are, herein, given for convenient reference. The F.sub.A center consists of one electron trapped in an anion vacancy adjacent to a substitutional cation impurity ion, i.e., an F center next to a cation impurity. The F.sub.B center consists of an F center adjacent to two cation impurities. The F.sub.2.sup.+ center is an electron trapped by two anion vacancies, i.e., a singly ionized pair of neighboring F centers along a &lt;100&gt; direction. If the F.sub.2.sup.+ center is adjacent to a substitutional cation impurity, e.g., Li.sup.+ or Na.sup.+ in a KCl crystal, it is referred to as a (F.sub.2.sup.+).sub.A center.
The F.sub.A and F.sub.B centers, though useful, are limited to wavelengths between 2.2 and 3.3 microns. Also, since these centers require visible region excitation, they undergo extremely large Stokes shifts, which cause considerable lattice heating. As a result, the maximum output powers of these lasers are on the order of 150 mw. Furthermore, these centers have relatively narrow emission bands, so that, three different crystals are required to cover this spectral range.
The intrinsic F.sub.2.sup.+ center in the presently used crystals, e.g., LiF and KF, can produce several watts of laser power very efficiently. Furthermore, depending on host lattice they have laser emission in the spectral range between 0.8 to 2.0 microns. Their principal drawback is that the technique which has, heretofore, been used to produce them required that they be maintained at low temperatures to avoid permanent loss.
An important new class of color-center materials use the emission of (F.sub.2.sup.+).sub.A centers for lasing in alkai halide crystals. The technique using these crystals as color-center lasers is disclosed and claimed in U.S. Pat. No. 4,301,426 by Irwin Schneider. Lasing is obtained from (F.sub.2.sup.+).sub.A centers which are dynamically stabilized by the presence of F.sub.A and F.sub.B centers which trap ((F.sub.2)-center electrons. This class of material provides laser action which is efficient, capable of pulsed or continuous operation, and broadly tunable, i.e., 1.6 to 1.9 microns and 2.0 to 2.5 microns using the (F.sub.2.sup.+).sub.A centers in Na or Li-doped KCl, respectfully. However each time a new range is covered a new crystal and new experimental set-up is required.
The breadth of tunability of a color-center material can be considerably increased; provided, it is possible to fabricate materials in which two or more usable centers can be made to co-exist in concentrations large enough to result in an efficient lasing over the entire tuning range. Further, these centers must have non-overlapping emission band in order to have the breadth of the total lasing to be approximately the sum of the widths of the emission of the individual centers. It is also important that the centers have overlapping absorptions; so that, only one pump source is required. If these problems could be solved, an efficient, broadly tunable solid state laser could be operated over the spectrum from wavelenths less than two microns to those greater than two microns.